Typical universal mold assemblies for manufacturing plastic containers include two half mold carriers each adapted to carry one half of a mold shell. Each mold shell half is interchangeable with a corresponding mold carrier to allow for different shaped articles to be manufactured by the mold assembly. The mold assembly includes a pair of support arms each supporting a corresponding carrier. The support arms are connected to a pivot point and act as levers to rotate about the pivot point to open and close the mold halves carried by the support arms through the carriers. Various latching and locking mechanisms have been developed to hold the carriers together during the forming operation. To allow for proper alignment of the mold shell during closure of the mold half carriers, one mold half carrier parting wall usually has bushings which mate with pins extending from the parting wall of the other mold half carrier. Mold assemblies of this configuration are described in U.S. Pat. No. 5,326,250 issued Jul. 5, 1994 to Doudement and published French Patent application Serial No. 2,659,265 published Sep. 17, 1991 assigned to Sidel S. A.
During the formation of the container in the mold shell, pressures as high as 600 pounds per square inch are forced into the cavity of the mold shell causing a plastic preform to expand into a container having a shape defined by the shape of the cavity of the mold shells. This formation pressure pushes the mold shells outwardly against the mold carriers and the supporting arms. Over time, these forces cause partial separation of the mold shells creating a flattened parting line defect on the plastic container formed in the mold cavity.
To compensate for parting of the mold shell halves during container formation, the aforementioned French patent application 2,659,265 has a single compensating surface area located on the outer surfaces of one of the mold half carriers. An air inlet orifice is contained in one of the support arms to direct air into this compensating surface area. The compensating surface area is approximately 15% greater than the surface area of the mold shell cavity. The compensating surface area is bounded by a continuous rectangular groove in which an O-ring is positioned to seal the periphery of the compensating surface area with an inside wall of the corresponding one support arm. The one support arm engaging the compensating surface area includes an air passageway communicating with the compensating surface area so that a compensating air pressure, in the order of 600 pounds per square inch, is forced evenly over the compensating surface area of this mold half carrier. This compensating air flow pressure pushes the mold carrier half against the expansion force associated with the air pressure being exerted outwardly from the mold cavity during container formation. As a result, the mold parts do not open during formation reducing or eliminating the formation of a parting line defect on the container.
The above described mold assemblies however do not provide for varying levels of air flow compensation pressure for different sizes of mold shell cavities. The counter pressure to the outside surface of the mold carrier should vary to accommodate for varying sizes of containers formed in the mold cavity. Compensating systems have been developed to address this problem by having multiple compensation surface areas located on the outside face of one of the carriers. Different air feeder lines are connected to the mold half carrier to supply pressurized air through the mold carrier to different groupings of the multiple compensation surface areas. That is a first feeder line passes to and through the carrier directly to one of the compensating areas while second and third feeder lines pass to and through the carrier to two other compensating areas. Control of air through the three feeder lines is located upstream of the carrier. The disadvantage with this multiple surface area air compensating system is that it requires the external or remote location of air flow controls having to be retrofitted to existing machines.
U.S. Pat. No. 6,099,286 issued Aug. 8, 2000 to Reinhold Nitsche, one of the inventors of the present invention, describes a mold carrier system that provides variable air compensation pressures for formation of bottles and containers of varying size. The mold carrier has two mold halves adapted to matingly engage each other and hold within the mold carriers interchangeable mold shells. The mold carriers each have an outside wall that has a plurality of independent air flow compensating surface areas located thereon. The air flow compensating surface areas are contained by a groove surrounding the flow compensating surfaces and an O-ring mounted in the grooves to prevent the escape of air away from the flow compensating surface areas. An air flow communication line interconnects the independent air flow compensating surface areas and includes shut off valves located in the carrier for controlling air flow between adjacent independent air flow compensating surface areas. The shut off valve has an actuator adapted to open and close the valve thereby connecting or disconnecting the independent air flow compensating surface areas located on opposite sides of the valve. The Nitsche variable air flow compensation system permits for adjustment of the compensating pressure applied to mold shells of differing cavity size during formation of a container in the cavity where the adjustment is readily accessible to an operator during a quick change over of the mold shells carried by the mold half carriers.
However, the prior art air flow compensating systems that employ an O-ring are subject to problems when the O-ring begins to wear over time. The O-ring is mounted in a rectangular groove surrounding the pressure compensating surface area. As the 600 pounds per square inch of air pressure is applied against the compensating pressure surface areas, the O-rings hold the air pressure in place. During each pressurization of the formation of the molds, the O-rings have a tendency to roll in the rectangular groove and wear down. Wearing of the O-ring results in air escaping away from the air compensating surface areas. Once the O-ring has degraded to a point where the escape of air is significant, then the pressure compensating effect of the mold carrier is lost. At this time the mold has to be shut down and the O-ring replaced. This unscheduled shut-down usually effects all mold carriers in the molding operation.
Accordingly, there is a need to provide for variable pressure compensation in mold carriers that is less susceptible to problems associated with O-ring wear.